The “metallocene compound” is well known as a homogeneous catalyst for olefin polymerization. Since the isotactic polymerization was reported by W. Kaminsky, et al. (Angew. Chem. Int. Ed. Engl., 24, 507 (1985)), there have been made many improvements in the olefin polymerization process using a metallocene compound, particularly a process for stereoregularly polymerizing an α-olefin. As an example of the improvement, a metallocene compound having a C2 symmetric structure wherein some hydrogen atoms of the cyclopentadienyl group in the ligand part are replaced with alkyl groups has been reported (by Yamazaki, et al., Chemistry Letters, 1853 (1989), Japanese Patent Laid-Open Publication No. 268307/1992). As like attempts, further, a large number of attempts to improve the isotactic stereoregularity of an olefin polymer by the use of a metallocene compound having, as a ligand, a bisindenyl derivative having a C2 symmetric structure have been reported (e.g., Angew. Chem. Int. Ed. Engl., 31, 1347 (1992), Organometallics, 13, 954 (1994)).
The metallocene compound of the C2 symmetric structure, however, is usually obtained as a mixture of a racemic modification and a mesoisomer, and only the racemic modification provides an isotactic polymer, while obtainable from the mesoisomer is only an atactic polymer, so that it is necessary to separate the racemic modification and the mesoisomer from each other in order to selectively obtain the isotactic polymer.
On the other hand, J. A. Ewen has found that an α-olefin is polymerized with syndiotactic stereoregularity by the use of a metallocene compound having a Cs symmetric structure wherein the cyclopentadienyl group and the fluorenyl group are bridged by dimethylmethylene (J. Am. Chem. Soc., 110, 6255 (1988)). To improve the metallocene compound, an attempt to introduce tert-butyl groups at the 2-position and the 7-position of the fluorenyl group and thereby further control the syndiotactic stereoregularity has been made (Japanese Patent Laid-Open Publication No. 69394/1992).
In addition, an attempt to synthesize an isotactic polymer by the use of a metallocene compound having a C1 symmetric structure that is different from the C2 symmetric and the Cs symmetric structures has been reported (see, for example, Japanese Patent Laid-Open Publications No. 193796/1991 and No. 122718/1994, EP 0881236).
The polymerization activities of these metallocene compounds, however, are still-insufficient, and hence development of metallocene compounds having excellent polymerization activities and olefin polymerization catalysts containing such metallocene compounds has been desired.
The metallocene compounds having Cs and C1 symmetric structures have an advantage in that the structural isomers such as a mesoisomer and a racemic modification are not produced, differently from the metallocene compound having a C2 symmetric structure.
Of the above metallocene compounds, the metallocene compound of the C1 symmetric structure, however, has a problem in that an unnecessary isomer wherein the substituent group is attached at a position different from the intended proper position is produced depending upon the preparation process. When such an isomer is used as, for example, an olefin polymerization catalyst, unfavorable results such as production of an atactic polymer as a by-product are often brought about. Hence, development of a process for selectively preparing a metallocene compound in which such an unnecessary isomer is not included has been desired.